The long term objectives of the proposed research are to identify and characterize novel proteins associated with oxygen adaptation in anaerobic methane-producing Archaea in order to provide a foundation of the function and evolution of oxygen detoxification enzymes. This investigation is important due to the relevance of detoxification of oxidative species as a general defense mechanism in all living cells, but also in the response of pathogens to oxidative burst produced by host defense cells. In addition, methane production is relevant to the problems of global warming and there is still much to learn about the physiology, biochemistry, and genetics of Archaea. The specific aims of this research proposal are: a) utilize a Methanosarcina acetivorans complete genome microarray to identify genes up-regulated during exposure of cells to oxygen, superoxide, and hydrogen peroxide, b) conduct transcriptional analysis and expression profiling of a previously identified gene cluster encoding probable oxidative stress proteins, including a superoxide reductase and rubredoxin: oxygen oxidoreductase (ROO), c) overexpress and purify the putative ROO from M. acetivorans to verify its catalytic properties in the context of the physiology of this organism.